Japanese Patent Laid Open Publication No. 8-87375 discloses a conventional strain detector. The conventional strain detector will be described with reference to drawings hereinafter. FIG. 9 is a top view of the conventional strain detector, and FIG. 10 is a cross sectional side view of the detector.
In FIG. 9 and FIG. 10, insulating substrate 1 made of elastic material is formed by disposing stick member 2 and insulating layer 3 thereon. Four strain-resistance elements 4 are disposed over insulating substrate 1. Strain-resistance elements 4 are electrically coupled to a pair of power electrodes 5, a pair of output electrodes 6, and a pair of ground (GND) electrodes 7 to form a bridge circuit. Protective layer 8 made of resins covers elements 4, power electrodes 5, the pair of output electrodes 6, the pair of GND electrodes 7 and the rest of insulating substrate 1
The operation of the above conventional strain detector will be described hereinafter.
When a shearing load is applied on the general-center position of the top of insulating substrate 1, a bending moment occurs in insulating substrate 1 via the shearing load and also occurs in the four strain-resistance elements 4 disposed over substrate 1. A resistance of strain-resistance elements 4 changes by the bending moment resulting in elements 4. A change of the resistance is supplied from the pair of output electrodes 6 to an external measuring device (not shown), and then the load on substrate 1 is measured.
In the conventional strain detector, only protective layer 8 made of resins is disposed over insulating substrate 1, the pair of power electrodes 5, the pair of output electrodes 6 and the pair of GND electrodes 7. Protective layer 8 made of resins absorbs water little by little. Therefore, when the strain detector is used for a long time in an atmosphere of high humidity, the water reaches strain-resistance elements 4 and the resistance of strain-resistance elements 4 fluctuates.